For the purpose of reducing costs, it is desirable for the production of single crystals to achieve the greatest possible crystal diameters and crystal lengths. This becomes particularly clear from the example of silicon single crystals which are pulled with a nominal diameter of 300 mm and a weight of more than 300 kg on an industrial scale according to the Czochralski method. Corresponding processes for producing single crystals having a nominal diameter of 450 mm are already being developed.
When a seed crystal is positioned on a melt, the thermal shock causes dislocations which have to be eliminated before the single crystal can be pulled. Dislocations have been proven to be eliminated by a comparatively thin neck portion initially being pulled, in the course of which dislocations are forced out of the crystal. In the case of heavy single crystals, in particular in the case of silicon single crystals which have a weight of more than 300 kg, the tensile strength in the neck portion is generally too weak to be able to withstand the load. It has therefore become accepted practise to relieve the neck portion before the load becomes critical. For this purpose, the neck portion is widened and tapered again so as to produce a widened portion between an upper and a lower neck portion. The lower neck portion is designed to be thicker than the upper neck portion and correspondingly has a greater tensile strength. For relieving the upper neck portion, use is made of a supporting device which engages below the widened portion and at least partially bears the weight of the single crystal. By way of example, the widened portion can be shaped in the manner of a bicone or a sphere.
US 2008/0022922 A1 describes a supporting device which comprises a supporting ring having an opening and gripping catches which are arranged opposite one another on the supporting ring and can be tilted about an axis. The gripping catches can execute a tilting movement from a waiting position into a holding position, in which they extend into the opening and under the widened portion. The supporting device has the disadvantage that the gripping catches drop irreversibly into the holding position. The undamped dropping of the dropping catches promotes the production of particles which threaten the continued single-crystal growth of the crystal. In addition, the gripping catches can no longer be moved back into the waiting position in order to be able to back-melt the crystal following the occurrence of dislocations.
In accordance with U.S. Pat. No. 5,843,229, a supporting device can also be embodied in such a way that it is possible to engage under the widened portion reversibly. This is achieved by means of a gripping device having two arms which intersect at a rotation point and are moved toward one another and away from one another by means of hydraulically operating cylinders. A disadvantage with this embodiment is that the arms move past one another in shearing fashion, and therefore a high degree of particle formation in the vicinity of the growing single crystal must be expected. Furthermore, the cylinders likewise produce particles during operation. However, the use of a shield, which holds back particles, to protect the single crystal is insufficient because the shield needs to have leadthroughs for the arms. In addition, the leadthroughs need to have a sufficient width so as to avoid obstructing the sideways movement of the arms.